Photochemistry has become an important area to consider in the semiconductor fabrication process. As an example, for the deposition of high temperature superconducting thin films, rapid isothermal processing (RIP) assisted metalorganic chemical vapor deposition (MOCVD) has provided the best low processing temperature results. This is due to the fact that incoherent radiation is the source of optical and thermal energy. Although the decomposition pathways of most of the reactions are not accurately known, in general, most of the organometallics used in MOCVD have higher adsorption coefficients in ultraviolet (UV) and vacuum ultraviolet (VUV) regions. High energy photons, from the UV and VUV regions, provide excited but not dissociated complexes. Thus, in concert with the low energy photons (responsible for the thermal or pyrolysis deposition), the high energy photons can provide the ideal MOCVD approach for the growth of many materials.
A photochemical reaction is a chemical reaction which takes place only under the influence of light. For the purposes of photochemistry, light is considered as being made up of individual photons of energy E=hv, where h is Planck's constant and v is the frequency of light. Only light that is absorbed can result in a photochemical effect. On the molecular scale the photochemical reaction starts with the adsorption of a photon by a molecule. The molecule is thereby promoted to an excited state. This excited molecule is a new chemical species which has its own distinct chemical and physical properties.
Any photochemical event starts with the absorption of a photon by a molecule M, with production of an excited molecule M*, where M+hv=M* (adsorption). The excited molecule M* may now react chemically, either by rearrangement or, for instance, by reaction with another species N: M*+N=P. This step, which chemically involves chemically the excited molecule M*, is the primary photochemical process.
Another type of molecule excitation is rotational excitation, which requires the smallest amount of energy. Rotational excitation results in a spinning of the molecule around a preferred axis, while the molecule is chemically unchanged. With higher energies, the molecule can be promoted to a vibrationally excited state. Here again the molecule is chemically unchanged as the energy is in the form of vibrations of various parts of the molecule. With even higher energies, the molecule will be electronically excited as one or several electrons are promoted to higher energy orbitals. Photochemical reactions occur from such electronically excited state of molecules
The ground state of the atom is the state in which all the electrons fill the available orbitals in the order of increasing energy. An electronically excited state is a state in which one or several electrons occupy higher energy orbitals, having left one or several vacancies in the lower orbitals. A ground state can only adsorb light, it cannot emit light. An excited state can either emit light (thus moving downwards in energy to the ground state or to a lower excited state) or absorb light (thus moving upwards in energy to a higher excited state).
In an article entitled "METALORGANIC CHEMICAL VAPOR DEPOSITION (MOCVD) OF OXIDES FOR ELECTRONIC AND PHOTONIC APPLICATIONS" by Singh et al., the authors teach how a VUV lamp greatly improves the quality of Y.sub.2 O.sub.3 films due to the use of higher energy photons in the deposition process. Singh et al. state: "Lamp configuration plays an important role in film deposition, which is very useful in the optimization of lamp heating source design. The leakage current density of Y.sub.2 O.sub.3 is good enough to replace SiO.sub.2 as the insulator in the devices processing. Good films can be grown at low substrate temperature by using VUV lamps as the source of optical energy."
While Singh et al. teach that a dielectric film's quality is greatly improved by the use of higher energy photons during deposition, the present invention develops a methodology to condition any dielectric film, no matter how it is deposited, by the use of higher energy photons so that the quality of the conditioned dielectric is greatly improved.